The present invention relates to an illuminator apparatus for the microlithographic manufacture of semiconductor integrated circuits and more particularly to illuminator apparatus which is operable at wavelengths of about 365 nanometers or less and which is protected against degradation by photopolymerization of volatile compounds associated with the lithographic resists.
As is understood by those skilled in the art, high density integrated circuits are typically manufactured by microlithographic techniques in which a semiconductor wafer coated with a photosensitive resist is exposed stepwise in a step-and-repeat projection system. At each step, an image of a mask or reticle is optically reduced and projected on the resist coated surface using light of a wavelength suitable for exposing the resist. To obtain improved resolution, the projection optics have been designed to utilize progressively shorter wavelengths as light sources and resists are developed. Currently, step-and-repeat systems are turning to the so-called i-line generated by high pressure mercury lamps. The i-line comprises wavelengths of about 365 nanometers as compared with the previously utilized g-line at 436 nanometers. As i-line step-and-repeat systems have been introduced into fab lines, however, they have suffered rapid deterioration in the output of the illuminator system. In accordance with one aspect of the present invention, it has been discovered that such degradation has not been caused by deterioration of the lamps or the optics associated with the lamps but rather has, in large part, been caused by the photopolymerization of volatile compounds associated with the resist materials and the depositing of the products of the photopolymerization on the surfaces of the optics. While photopolymerization of such volatile compounds has probably occurred previously to varying extents, it was generally unrecognized and did not create a significant problem. In accordance with another aspect of the present invention, it has been discovered that typical reaction products of the photopolymerization process described above are highly absorbtive at 365 nanometers but are relatively transmissive at only slightly longer wavelengths. In accordance with another aspect of the invention, it has been found that the offending volatile compounds can be removed from the cooling air stream supplied to the illuminator by passing it over a bed of activated carbon particles. A particulate filter downstream from the activated carbon bed prevents particulates from being drawn into the illuminator system.
Among the several objects of the present invention may be noted the provision of novel illuminator apparatus for the microlithographic manufacture of semiconductor integrated circuits; the provision of such illuminator apparatus which is operable at wavelengths of about 365 nanometers or less; the provision of such apparatus which utilizes a high pressure mercury arc lamp as a light source; the provision of such illuminator apparatus which does not experience degradation of the optical elements associated with the light source; the provision of such illuminator apparatus which provides highly uniform illumination; the provision of such apparatus which is highly reliable and which is of relatively simple and inexpensive manufacture. Other objects and features will be in part apparent and in part pointed out hereinafter.